The present invention relates generally to an image-formation optical system well fit for use with imaging systems using solid-state imaging devices such as CCDs or CMOSs, and more particularly to an image-formation optical system used with onboard cameras, surveillance cameras, digital cameras and digital video cameras as well as small-format cameras mounted on cellular phones, personal computers, etc., and an imaging system that incorporates the same.
In recent years, cars with onboard cameras have begun to catch on. And early onboard cameras have been used primarily for visual identification; in more recent years, however, they have taken the form of more intelligent sensor cameras taking aim at sensing white lines, monitoring whether or not cars stay on lanes, finding something abnormal, and keeping watch on drivers. Requirements for optical systems used with such cameras are: a wide angle of view, specifically a half angle of view of 90° or greater to get rid of any dead angle, higher image quality, and a compact, low-cost arrangement comprising fewer lenses. Fisheye optical systems are thus now used.
Some prior art fisheye optical systems comprising fewer lenses, say, six lenses are available, as set forth in Patent Publications 1 and 2.
However, the prior arts disclosed there are found to have the following problems.
Although the prior arts have a half angle of 90° or greater with a relatively small number of lenses, say, six lenses, yet the optical effective diameter of the first lens grows large with respect to image height. Consequently, the fabrication cost of the first lens soars and it is difficult to reduce the whole size of the system.
In general, for a wide-angle optical system there is a retrofocus type used that comprises, in order from its object side, a lens group having negative refracting power and a lens group having positive refracting power. When such an optical system is used at a superwide-angle supposed to be 90° or greater in the half angle of view, it has to have a short focal length. In other words, it is required that the power of the lens group having negative refracting power be strengthened to shift the principal point position to an image plane side. As the power of the lens group having negative refracting power grows strong, there will be a lot more aberrations occurring such as field curvature and astigmatism.
To stave off such aberrations, the front group of negative refracting power is generally made up of a plurality of lenses thereby dispersing and reducing them. To this end, it is general that two negative lenses be put one upon another, as set forth typically in Patent Publication 1.
However, the aforesaid optical system, because of comprising a lot more optical lenses, has a longer whole length. The fisheye lens of the retrofocus type constructed as described above, because of an increased optical effective diameter of the front group lens, ends up with increased lens diameter, and high prices as well.
Patent Publication 1
U.S. Pat. No. 2,992,547
Patent Publication 2
JP(A)2002-72085